Personally I think there will be a cut-off point where SSD drives fit certain demographics, and manufacturers follow suit. Investing R&D into larger platter drives won't be cost effective for them past that point.
This point is probably where SSD drives that are "big enough" for most users (say, around 300G) and also reach a price that "most users" can afford. I'd guess that point is around £200. Harddrives will be relegated to "cheap, slow, big storage" and be limited to mostly external backup devices. Harddrive manufacturers will push into the new SSD territory hard, after all, they have new economies of scale and profits to be made from them. Platter based HDs must be really tight.
SSDs are easily marketable to regular users - they are faster, quieter, more reliable, possibly lighter (good for laptops), lower power consumption. They have already been making inroads on high end laptops for the last year or so. They are now already the same capacity as most laptop drives, certainly when Intel bring out their 320G one they will be.
I think SSDs will continue marching into laptop land, and in about a year, 18 months tops, they will dominate that area. The manufacturers are smart, that's why they've all been making them 2.5" scale. This will make the drives cheaper, then they'll move more into the desktop market. I think the change will be quite fast, 3 years tops, for most PCs to be sold with SSDs, with platter drives being relegated to backup devices.